This invention relates to a method of making low dusting joint compounds. More specifically, it relates to a method of more efficiently adding the dedusting agent to the product.
In the construction of buildings, one of the most common building elements is gypsum wallboard, often known as drywall or gypsum paneling, used in the construction of walls and/or ceilings. The board may be composed of any of a variety of materials, including but not limited to, cementitious materials such as, for example, cement or gypsum. The term “drywall” is used illustratively, but as one of ordinary skill will appreciate, includes a variety of products such as wallboard, fiberboard, glass-faced gypsum boards, cement board, and the like. Walls made from gypsum wallboard are traditionally constructed by affixing the panels to wood studs or metal framing, and treating the joints between adjoining panels with a specially prepared adhesive called a joint compound. A huge advantage of this system is the ability to cut the gypsum panels to custom sizes and shapes. Gypsum panels easily accommodate walls that are unusual in size and can be shaped around structural elements such as beams or pipes. The panels are generally cut with a mat knife to the desired shape, joined to a substrate, and then finished with a joint compound. The side edges of the drywall panels are tapered, thus allowing the joint compound to be applied to the seam, between adjoining panels, in such a way that a monolithic surface is created when finished. It is well known in the art that finishing a joint between boards involves three steps. First a thin layer of joint compound is applied to the boards over the joint, and a liquid-permeable paper or fiberglass tape is embedded into it. This step is commonly referred to as the embedding step. Next, a second coat of joint compound is applied over the embedded joint tape. This step is commonly referred to as the fill step. The second coat of joint compound typically extends approximately two inches beyond the edges of the joint tape. Finally a third coat of joint compound is applied over the first two coats, where the third coat typically extends even further out from the edges of the joint tape. Both the second and third coat may be subsequently lightly sanded upon drying. Joint compounds are also used to make repairs of defects, such as uneven surfaces, holes, depressions, gaps, dents, and other imperfections including those around electrical boxes, piping and duct work, as well as corners created by the intersection of drywall boards.
There are several categories of joint compounds. Drying type compounds cure through the evaporation of water, whereas setting type joint compounds chemically react with water during the curing process. Setting type joint compounds typically use calcium sulfate hemihydrate, also known as stucco or plaster of Paris, as a base. When water is added to the setting type powder, it reacts with the calcium sulfate hemihydrate via a hydration reaction to form an interlocking matrix of calcium sulfate dihydrate crystals. The interlocking crystal matrix gives the compound increased strength. The benefit of a setting type joint compound over a drying type is the overall strength of the finished joint, resulting in less shrinking and cracking, as well as an independence from having to wait for the joint compound to be completely dry prior to further finishing. Drying type joint compounds typically use calcium carbonate as a base and cure through drying. Drying type joint compounds have the advantage of ease of use, as they typically come in a ready mixed form, with water being added and mixed by the manufacturer. A third type of joint compound combines the setting action of a calcium sulfate hemihydrate based compound with the ease of use of a ready mixed compound. The properties of a ready mixed setting type joint compound are taught in U.S. Pat. No. 5,746,822, incorporated herein by reference.
Between coats of joint compound, and before the wall is decorated, it is necessary to sand the joint compound to even the surface. While doing so, the dust generated is generally very fine and tends to remain airborne, settling out only after long periods of time. This long residence time in the air allows it to travel long distances from the site of the wall. In a home repair, this fine dust is not confined to the area immediately adjacent to where the joint compound is being used, but is often found throughout the house, resulting in fine dust settling everywhere.
Liquid mineral oils are known to reduce dust in gypsum-based compositions. However, these additives have a number of drawbacks. When such liquids are mixed into the composition, they tend to migrate toward the surface of the gypsum product, giving an uneven distribution of dust reducing properties. Whereas dust reduction may be acceptable when friction is applied to the surface of the product, the hardened product may have very different properties when finishing extends into the product interior. This may cause the product to dry unevenly as well. Buildup of oils at the product surface may inhibit passage of water vapor from the interior of the product. Oil and liquid waxes also cause the loss of adhesion to the substrate at higher concentrations.
Solid waxes have also been used to reduce the amount of gypsum fines generated by sanding or abrading of gypsum surfaces. Choice of a solid wax was made to overcome the problems with liquid compounds and so that the wax could be conveniently added with the solid components during wallboard manufacture. However, the use of high molecular weight solid waxes alone results in unsatisfactory dust retrieval during cutting of gypsum panels using hand-held tools. While not wishing to be bound by theory, it is believed that there is less friction generated by a hand-held mat knife compared to a power tool, thereby limiting the amount of heat available to melt the wax and agglomerate the fines.
Although some synthetic waxes are water soluble, they are not necessarily easy to solubilize. Some synthetic waxes are in the form of very fine powders that are difficult to wet, and once wet, dissolve very slowly. When added to the slurry water with other solid components, the solid synthetic waxes tend to float on the surface for a long time, dissolving slowly after prolonged mixing.
Even use of soft solid waxes poses problems. Materials that are too soft cannot easily be fed through the dry additive system with other solid additives. Polyethylene glycol (“PEG”) having a molecular weight of 750 Daltons is a soft solid synthetic wax that is known to clog the transfer equipment in a dry additive system. Although PEG 750 is an excellent dedusting agent, adding it to a joint compound composition from the dry additive system is unsatisfactory. Modification of the manufacturing process to add the soft solid directly to the process water even fails to solve the problem under some conditions. Although synthetic waxes are water soluble, they take a long time to liquefy under plant conditions. In a manufacturing plant in the northern part of the United States in December, temperatures can average below 50° F. (10° C.). At these temperatures, PEG 750 can take up to 5 days to melt and dissolve by mixing in water.
Melting the soft solid to form a liquid also fails to alleviate all of the problems described above. It is difficult to maintain constant temperatures in large manufacturing facilities due to high ceilings, large volumes and constant opening and closing of doors or loading facilities. Where, as with PEG 750, the melting temperature is close to room temperature, once the soft solid is melted, the room temperature may drop below the freezing point of the dedusting agent, causing it to harden again. Heat may have to be constantly applied to the PEG 750 to keep it in a liquid form or it may need to be melted multiple times as temperatures vary. In either case, energy is used in many climates to keep the dedusting agent in the liquid form.
There is, therefore, a need for an improved method of adding soft solid dedusting agents to a slurry that allows for dispersing of the dedusting agent in a reasonable time. There is a further need for a method of making a joint compound slurry with a dedusting agent that is economically manufactured at low temperatures.